Method for coating molded articles

Abstract

This invention discloses a coating method of plastic molded articles, which can form multilayer coating film composed of three layers of water-based primer, water-based base paint and clear paint, the film excelling in adherability to plastic molded articles, water resistance and durability, even when low-temperature baking at not higher than 100° C. is carried out in 3-coat-1-bake system.

1 . A coating method of plastic molded articles comprising applying a water-based primer (I) onto a plastic molded article, applying onto the unhardened coated surface a water-based base coat paint (II), further applying onto the unhardened coated surface a clear coat paint (III), and simultaneously baking and hardening the so formed three-layered coating film, characterized in that
 the water-based primer (I) contains an aqueous polyolefin resin (A) and an aqueous acrylic resin (B) at such ratios that the resin (A) is 10-90 wt % and the resin (B) is 90-10 wt %, based on the total solid content of the two resins,   the water-based base coat paint (II) contains an aqueous polyurethane resin (C), hydroxyl-containing aqueous acrylic resin and/or aqueous polyester resin (D), and a melamine resin (E), at such ratios that the resin (C) is 1-50 wt %, the resin (D) is 10-96 wt % and the resin (E) is 3-40 wt %, based on the total solid content of these three resins, the melamine resin (E) being a butyl/methyl mixed etherified melamine resin and having a weight-average molecular weight within a range of 1,500-3,000, and   the clear coat paint (III) contains a hydroxyl-containing resin (F) and an isocyanate crosslinking agent (G), the isocyanate crosslinking agent (G) containing a polyisocyanate compound having urethodione structure and at least trimer of a diisocyanate compound, at a solid weight ratio within a range of 10/90-50/50.   
     
     
         2 . A method according to  claim 1 , in which the aqueous polyolefin resin (A) is obtained by graft polymerization of polymerizable unsaturated dicarboxylic acid or anhydride thereof to an olefinic polymer. 
     
     
         3 . A method according to  claim 1 , in which the aqueous acrylic resin (B) is a carboxyl-containing acrylic resin or a carboxyl-containing acrylic resin emulsion. 
     
     
         4 . A method according to  claim 1 , in which the water-based primer (I) contains the aqueous polyolefin resin (A) and aqueous acrylic resin (B) at such ratios that the resin (A) is 40-80 wt % and resin (B) is 60-20 wt %, based on the total solid content of the two resins. 
     
     
         5 . A method according to  claim 1 , in which the aqueous polyurethane resin (C) is an aqueous dispersion of self-emulsifying type urethane resin having an average particle size of 0.001-1.0 μm, which is prepared by the steps of reacting (i) aliphatic and/or alicyclic diisocyanate, (ii) diol having a number-average molecular weight of 500-5,000, (iii) low molecular weight polyhydroxyl compound and (iv) dimethylolalkanoic acid, at an NCO/OH equivalent ratio within a range of 1/0.5-1/0.95, by one-shot process or multistage process; and chain-extending and emulsifying the resulting urethane prepolymer, either after or during neutralization. 
     
     
         6 . A method according to  claim 1 , in which the hydroxyl- containing aqueous acrylic resin and/or aqueous polyester resin (D) is an aqueous acrylic resin having an acid value of 10-100 mgKOH/g and a hydroxyl value of 10-250 mgKOH/g and/or an aqueous polyester resin having an acid value of 10-100 mgKOH/g and a hydroxyl value of 10-300 mgKOH/g. 
     
     
         7 . A method according to  claim 1 , in which the melamine resin (E) is a butyl/methyl mixed etherified melamine resin, the mol ratio of the butyl ether group/methyl ether group therein being within a range of 4/6-9/1. 
     
     
         8 . A method according to  claim 1 , in which the water-based base coat paint (II) comprises aqueous polyurethane resin (C), hydroxyl-containing aqueous acrylic resin and/or aqueous polyester resin (D) and melamine resin (E) at such ratios, based on the total solid content of these three resins, that the resin (C) is 3-30 wt %, the resin (D) is 30-80 wt %, and the resin (E) is 10-30 wt %. 
     
     
         9 . A method according to  claim 1 , in which the hydroxyl-containing resin (F) is a hydroxyl-containing acrylic resin. 
     
     
         10 . A method according to  claim 1 , in which the isocyanate crosslinking agent (G) has a viscosity not higher than 1000 mPa·s/25° C. 
     
     
         11 . A method according to  claim 1 , in which the clear coat paint (III) contains the hydroxyl-containing resin (F) and the isocyanate crosslinking agent (G) at such ratios that 0.8-2.0 equivalent of isocyanate groups in the crosslinking agent (G) are present per equivalent of hydroxyl groups in the resin (F). 
     
     
         12 . A method according to  claim 1 , in which the three-layered coating film is baked at a temperature within a range of 40-110° C. for 5-60 minutes. 
     
     
         13 . Coated articles obtained by the method according to  claim 1 . 
     
     
         14 . Coated articles obtained by the method according to  claim 2 . 
     
     
         15 . Coated articles obtained by the method according to  claim 3 . 
     
     
         16 . Coated articles obtained by the method according to  claim 4 . 
     
     
         17 . Coated articles obtained by the method according to  claim 5 . 
     
     
         18 . Coated articles obtained by the method according to  claim 6 . 
     
     
         19 . Coated articles obtained by the method according to  claim 7 . 
     
     
         20 . Coated articles obtained by the method according to  claim 8 . 
     
     
         21 . Coated articles obtained by the method according to  claim 9 . 
     
     
         22 . Coated articles obtained by the method according to  claim 10 . 
     
     
         23 . Coated articles obtained by the method according to  claim 11 . 
     
     
         24 . Coated articles obtained by the method according to  claim 12 .